I love my Beta 57a mics for live vocals... really crisp and clear (if not as warm). But I've had less luck with them on guitar cabs... they sound okay, and are better backed off the cab a foot or so, but I love the sound of a regular 57 on a guitar cab. You'll be hard-pressed to find a mic that's equally suited to both purposes... maybe an Audix OM-7 ($$$).

I have a Beta 57A and don't really care for it when put on a guitar cab for recording purposes. It's a little too bright for my taste. The other mic I have is a Sennheiser 421, shich I prefer to even a 57 I think. Warm and fat with bite too!

I've spent hours comparing both mics. I like the old 57's for a tighter low end. Almost a more solid type sound. The Beta has a warmer hi-end & a bit smoother midrange.
But that's just me on my system. My old 57's are over 15 years old so they may not be as good as a newer one. My Beta 57's brand new though. I plan to use the old 57's on most of my recordings.

The Betas get a bad rap as far as thier Recording sound gose. They are intended for Live Performance. They have some Bass rolled off to avoid low end stage feedback. They have some additional Prescense added to cut through the mix in a Live Performance.

In a recording situation its eay to compensate for this by rolling just a little bit of Highs Off and Boost the Bass just a little bit.

I'm currently sporting 3 Beta 57's, 1 Beta 58, 1 Beta 87, 1 ASX5 and 1 Sennheiser E609 which I'm having great results with. Of coarse I have all the old mics too which I break out for drums.